Keyword Spy is a keyword research tool that allows you to research the keywords that your competitors are using in their advertising campaigns. Keyword Spy operates as a web based system, and works with Google AdWords, Bing/Microsoft Adcenter and Yahoo Search Marketing. It gives you the ability to quickly select the best keywords for your advertising campaigns.
To use Keyword Spy, a membership is required. The Keyword Spy people offer a lifetime free trial to get you started.